Discovery of a directly imaged planet to the young solar analog
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Astronomy & Astrophysics manuscript no. ajbohn_yses_companion_tyc8984 ©ESO 2021
April 22, 2021
Discovery of a directly imaged planet to the young solar analog
YSES 2?
Alexander J. Bohn1 , Christian Ginski2, 1 , Matthew A. Kenworthy1 , Eric E. Mamajek3, 4 , Mark J. Pecaut5 ,
Markus Mugrauer6 , Nikolaus Vogt7 , Christian Adam7, 8 , Tiffany Meshkat9 , Maddalena Reggiani10 , and Frans Snik1
1
Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: bohn@strw.leidenuniv.nl
2
Sterrenkundig Instituut Anton Pannekoek, Science Park 904, 1098 XH Amsterdam, The Netherlands
3
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, M/S 321-100, Pasadena CA 91109, USA
4
Department of Physics & Astronomy, University of Rochester, Rochester NY 14627, USA
arXiv:2104.08285v2 [astro-ph.EP] 21 Apr 2021
5
Rockhurst University, Department of Physics, 1100 Rockhurst Road, Kansas City MO 64110, USA
6
Astrophysikalisches Institut und Universitäts-Sternwarte Jena, Schillergäßchen 2, D-07745 Jena, Germany
7
Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Bretaña 1111, Playa Ancha, Val-
paraíso, Chile
8
Núcleo Milenio Formación Planetaria - NPF, Universidad de Valparaíso, Av. Gran Bretaña 1111, Valparaíso, Chile
9
IPAC, California Institute of Technology, M/C 100-22, 1200 East California Boulevard, Pasadena CA 91125, USA
10
Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium
Received February 6, 2021 / Accepted
ABSTRACT
Context. To understand the origin and formation pathway of wide-orbit gas giant planets, it is necessary to expand the limited sample
of these objects. The mass of exoplanets derived with spectrophotometry, however, varies strongly as a function of the age of the
system and the mass of the primary star.
Aims. By selecting stars with similar ages and masses, the Young Suns Exoplanet Survey (YSES) aims to detect and characterize
planetary-mass companions to solar-type host stars in the Scorpius-Centaurus association.
Methods. Our survey is carried out with VLT/SPHERE with short exposure sequences on the order of 5 min per star per filter. The
subtraction of the stellar point spread function (PSF) is based on reference star differential imaging (RDI) using the other targets
(with similar colors and magnitudes) in the survey in combination with principal component analysis. Two astrometric epochs that
are separated by more than one year are used to confirm co-moving companions by proper motion analysis.
Results. We report the discovery of YSES 2b, a co-moving, planetary-mass companion to the K1 star YSES 2 (TYC 8984-2245-1,
2MASS J11275535-6626046). The primary has a Gaia EDR3 distance of 110 pc, and we derive a revised mass of 1.1 M and an age
of approximately 14 Myr. We detect the companion in two observing epochs southwest of the star at a position angle of 205◦ and
with a separation of ∼ 100. 05, which translates to a minimum physical separation of 115 au at the distance of the system. Photometric
measurements in the H and K s bands are indicative of a late L spectral type, similar to the innermost planets around HR 8799. We
derive a photometric planet mass of 6.3+1.6 −0.9 MJup using AMES-COND and AMES-dusty evolutionary models; this mass corresponds
to a mass ratio of q = (0.5 ± 0.1) % with the primary. This is the lowest mass ratio of a direct imaging planet around a solar-type star
to date. We discuss potential formation mechanisms and find that the current position of the planet is compatible with formation by
disk gravitational instability, but its mass is lower than expected from numerical simulations. Formation via core accretion must have
occurred closer to the star, yet we do not find evidence that supports the required outward migration, such as via scattering off another
undiscovered companion in the system. We can exclude additional companions with masses greater than 13 MJup in the full field of
view of the detector (000. 15 < ρ < 500. 50), at 000. 5 we can rule out further objects that are more massive than 6 MJup , and for projected
separations ρ > 200 we are sensitive to planets with masses as low as 2 MJup .
Conclusions. YSES 2b is an ideal target for follow-up observations to further the understanding of the physical and chemical forma-
tion mechanisms of wide-orbit Jovian planets. The YSES strategy of short snapshot observations (≤ 5 min) and PSF subtraction based
on a large reference library proves to be extremely efficient and should be considered for future direct imaging surveys.
Key words. Planets and satellites: detection — Planets and satellites: formation — Instrumentation: high angular resolution —
Techniques: image processing — Stars: individual: YSES 2 (TYC 8984-2245-1)
1. Introduction past few years (e.g., Marois et al. 2008; Schmidt et al. 2008;
Marois et al. 2010; Lagrange et al. 2010; Rameau et al. 2013;
Despite several remarkable exoplanet and brown dwarf discov- Bailey et al. 2014; Macintosh et al. 2015; Chauvin et al. 2017;
eries by high-contrast imaging at high angular resolution in the Keppler et al. 2018; Haffert et al. 2019; Janson et al. 2019; Bohn
?
Based on observations collected at the European Organisa- et al. 2020a,b), there is an ongoing debate regarding the forma-
tion for Astronomical Research in the Southern Hemisphere under tion mechanisms that create these super-Jovian gas giants with
ESO programs 099.C-0698(A), 0101.C-0153(A), 0101.C-0341(A), and semimajor axes greater than 10 au. It is unclear whether these
106.20X2.001. companions have a star-like origin from a collapsing molecu-
Article number, page 1 of 15
A&A proofs: manuscript no. ajbohn_yses_companion_tyc8984
lar cloud that is broken up into fragments, creating planetary- In this article we report the detection of a new exoplanet that
mass objects similar to a stellar binary (Kroupa 2001; Chabrier was discovered within the scope of our survey. As this is already
2003), or through formation in a circumstellar disk instead. The the second planetary system discovered by YSES, we introduce
classical bottom-up framework postulates formation via core ac- a new stellar identifier that is based on our survey acronym. The
cretion by coagulation of small dust grains into planetary em- details of this new designation are described in §2. In §3 we
bryos (Pollack et al. 1996; Alibert et al. 2005; Dodson-Robinson describe our observations and data reduction methods. We dis-
et al. 2009; Lambrechts & Johansen 2012). These evolve either cuss previous observations on the host star and reassess its main
via collisions or pebble accretion (Johansen & Lacerda 2010; parameters in §4. The results of our high-contrast imaging ob-
Ormel & Klahr 2010) into planetary cores that are massive servations are presented and analyzed in §5. We discuss poten-
enough to accrete a gaseous envelope and to open a gap in the tial formation mechanisms of this newly detected exoplanet in
disk (Paardekooper & Mellema 2004). In the corresponding top- §6 and we present our conclusions in §7.
down scenario, planetary cores can be created by gravitational
instabilities leading to the collapse of dense regions in the pro-
toplanetary disk (Boss 1997; Rafikov 2005; Durisen et al. 2007; 2. Nomenclature of YSES planets
Kratter et al. 2010; Boss 2011; Kratter & Lodato 2016). Owing to the recent success of YSES, we decided to introduce
To study this question for the underlying planet forma- a dedicated catalog that will be used for star-planet systems dis-
tion mechanisms from a statistical point of view, several direct covered within the scope of our survey. The YSES acronym has
imaging surveys have been conducted (e.g., Vigan et al. 2012; been verified by the IAU Commission B2 Working Group on
Galicher et al. 2016; Bowler 2016; Vigan et al. 2017). Synthetic Designations and was added to the Simbad database (Wenger
planet populations that represent each of the potential formation et al. 2000).1 The nomenclature of planet hosts from our survey
channels can be compared to the observational results from sur- is YSES NNN and planets that are associated with these stars
veys and place constraints on the efficiency of the correspond- will be named YSES NNNa, accordingly. Following these guide-
ing formation pathway (e.g., Mordasini et al. 2009a,b; Forgan & lines, we assigned the host star of the intriguing multi-planet
Rice 2013; Forgan et al. 2018). The two largest surveys were car- system that was discovered around TYC 8998-760-1 the new
ried out with two of the most advanced adaptive-optics assisted, primary identifier YSES 1 (Bohn et al. 2020a,b). The planets
high-contrast imagers available: the Spectro-Polarimetric High- formerly known as TYC 8998-760-1 b and TYC 8998-760-1 c,
contrast Exoplanet REsearch (SPHERE; Beuzit et al. 2019) in- will be named YSES 1b and YSES 1c, henceforth. Further plan-
strument at the 8.2 m ESO/VLT and the Gemini Planet Im- etary systems discovered by our survey will receive designated
ager (GPI; Macintosh et al. 2014) at the 8.1 m Gemini South YSES identifiers followed by ascending integer identifications
telescope. The preliminary statistical analysis of the first 300 (IDs). Hence, the new companion discovered within the scope
stars from the Gemini PLanet Imager Exoplanet Survey (GPIES; of this paper will be referred to as YSES 2b, orbiting its Sun-like
Nielsen et al. 2019) concludes that giant planets between 10 au host YSES 2.
and 100 au that have masses smaller than 13 MJup favorably form
via core accretion mechanisms, whereas brown dwarf compan- 3. Observations and data reduction
ions in the same separation range but with masses from 13 MJup
to 80 MJup seem to be predominantly created by disk instabil- We observed YSES 2 (TYC 8984-2245-1, 2MASS J11275535-
ities. This finding is supported by the analysis of the first 150 6626046) as part of YSES on the nights of 2018 April 30 (PI:
stars observed within the scope of the SpHere INfrared survey Kenworthy) and 2020 December 8 (PI: Vogt) with SPHERE
for Exoplanets (SHINE; Vigan et al. 2020), which additionally (mounted at the Nasmyth platform of Unit Telescope 3 of the
hypothesizes that companions with masses between 1 MJup and ESO Very Large Telescope). We used the IRDIS camera (Dohlen
75 MJup are likely to originate from bottom-up formation sce- et al. 2008) in classical imaging mode, applying a broadband fil-
narios around B and A type stars, whilst objects of the same ter in the H and K s bands during the first and second nights, re-
mass around M-type stars are consistent with simulated popula- spectively. The observations were carried out in pupil stabilized
tions from top-down mechanisms. For the intermediate masses imaging mode and an apodized Lyot coronagraph was used to
of F-, G-, and K-type stars, the observed detections can be ex- block the flux of the primary star (Soummer 2005; Martinez et al.
plained by a combination of both formalisms. A statistical meta- 2009; Carbillet et al. 2011). In addition to the science frames, we
analysis on the distribution of wide-orbit companion eccentrici- obtained center frames, with a sinusoidal pattern applied to the
ties carried out by Bowler et al. (2020) provides supporting ev- deformable mirror that creates a waffle pattern to locate the posi-
idence for two distinct formation channels shaping the popula- tion of the star behind the coronagraphic mask; sky frames of an
tions of giant planets (2 MJup < M < 15 MJup ) and brown dwarfs offset position with no adaptive optics (AO) correction and with-
(15 MJup < M < 75 MJup ). out any source in the field of view, to subtract the instrument and
thermal background; and non-coronagraphic images of the star
Most of these statistical evaluations are affected by the small that are used for photometric reference of point sources detected
number of actual substellar companions that were detected in in the science images. This last category of non-coronagraphic
the preceding imaging surveys. To expand the sample size for flux images was obtained with an additional neutral density filter
solar-mass host stars, we started the Young Suns Exoplanet Sur- in the optical path to record an unsaturated stellar point spread
vey (YSES; Bohn et al. 2020a, Bohn et al. in prep.) that is ob- function (PSF) in the linear readout regime of the detector. This
serving a homogeneous sample of 70 ∼15 Myr-old, K-type stars neutral density filter (filter ID: ND_1.0) provided an attenuation
in the Lower Centaurus Crux (LCC) subgroup of the Scorpius- of 7.9 and 6.9 across the H and K s bandpasses, respectively. A
Centaurus association (Sco-Cen; de Zeeuw et al. 1999). All stars detailed description of the observing setup and the weather con-
have masses close to 1 M and the proximity (average parallac- ditions can be found in Appendix A.
tic distance hDi = 114 ± 17; Gaia Collaboration et al. 2020) and
youth of the LCC facilitate the direct imaging search of giant, 1
Database entry available at: http://cds.u-strasbg.fr/
self-luminous substellar companions around these stars. cgi-bin/Dic-Simbad?/18721212
Article number, page 2 of 15Alexander J. Bohn et al.: Discovery of a directly imaged planet to the young solar analog YSES 2
The data reduction was performed with PynPoint (version to be a variable in the All Sky Automatic Survey (ASAS; IDed as
0.8.1; Stolker et al. 2019) and included basic processing steps ASAS J112755-6625.9) showing high amplitude (0.093 mag in
such as dark and flat calibration, bad pixel cleaning, sky sub- V) and rapid rotation (Prot = 2.7325 d), which is consistent with
traction, and correction for the instrumental distortion along the the observed saturated X-ray emission (and right near the median
vertical axis of the detector. To remove the stellar halo that is rotation period for Sco-Cen pre-main-sequence stars of hProt i '
affecting approximately the innermost 100. 2 around the coron- 2.4 d; Mellon et al. 2017). Pecaut & Mamajek (2016) include
agraph, we utilized an approach based on reference star dif- the star in their age analysis of pre-main-sequence K stars across
ferential imaging (RDI; Smith & Terrile 1984) in combination Sco-Cen, estimating an age of 23 Myr and a mass of 1.0 M .
with principal component analysis (PCA; Amara & Quanz 2012; The star has subsequently appeared in multiple LCC member-
Soummer et al. 2012). As the parallactic rotation of our YSES ship lists (Gagné et al. 2018; Goldman et al. 2018; Damiani et al.
observations is usually less than a few degrees, classical PSF 2019). The status of this star as a pre-main-sequence member of
subtraction schemes, such as angular differential imaging (ADI; LCC is strongly corroborated by a 99.9 % membership probabil-
Marois et al. 2006), perform much worse compared to this ref- ity from the BANYAN Σ algorithm (Gagné et al. 2018) applied
erence library approach. This method of combined RDI plus to the available Gaia astrometry and radial velocities (Gaia Col-
PCA was already successfully employed to recover circumstel- laboration et al. 2018, 2020).
lar disks in archival HST data (e.g., Choquet et al. 2014) and
within the scope of our survey for the discovery of a transition
4.2. Spectral analysis of YSES 2
disk around the YSES target Wray 15-788 (Bohn et al. 2019).
Owing to the same location on sky, similar distances, and To check the previously published spectral properties of YSES 2,
spectral types, all YSES targets exhibit very similar magnitudes we examined two archival UVES spectra from the ESO archive
in the red part of the optical spectrum (where the wavefront taken on UT 2007 May 2 (Program 079.C-0556(A); PI Torres).
sensor of SPHERE is operating) and at the near-infrared wave- The UVES spectra at resolution R = 40,000 were convolved to
lengths of our scientific observations. This facilitates compara- lower resolution R = 3,000 and compared to the grid of MK spec-
ble AO corrections amongst all our YSES observations, and the tral standards from Pecaut & Mamajek (2016). The blue spec-
resulting images compose an excellent reference library to per- trum, 3280 Å– 4560 Å, is consistent with K0V, but the red spec-
form RDI. The reference targets that were used for our library trum, 4730 Å– 6840 Å, appears to be K2V. The Hα line exhibits
PSF subtraction are listed in Appendix B. We modeled the stel- marginal emission, similar to the filled-in emission reported by
lar PSF with 50 principal components that were obtained from Torres et al. (2006). Hence, we infer a temperature type of K1±1
our full reference library. After the PSF subtraction, the frames and confirm the type K1V(e) published by Torres et al. (2006).
were de-rotated according to their parallactic angles and median From the original spectra, we independently measure the equiv-
combined. alent width of the Li I λ6707 feature to be 364 ± 5 mÅ, by simul-
For the astrometric calibration we used the standard in- taneously fitting Voigt profiles to the Li I feature and Fe I blend
strumental solution as presented by Maire et al. (2016) with nearby (see, e.g., Soderblom et al. 1993). This is in good agree-
a wavelength independent true north offset of −1◦.75 ± 0◦.08 ment with the 367 mÅ reported by Torres et al. (2006). The as-
and plate scales of (12.251 ± 0.010) mas px−1 and (12.265 ± signed luminosity class that we adapted from Torres et al. (2006)
0.010) mas px−1 in the H and K s band, respectively. does not necessarily imply that YSES 2 is on the main sequence
rather than being a pre-main-sequence star. Even though the lu-
minosity and gravity indicators used by Torres et al. (2006) were
4. Stellar properties more in line with main-sequence dwarfs than subgiant or giant
We briefly summarize previous literature characterizing YSES 2 standard stars, more persuasive indicators such as the HRD po-
in §4.1 and compile the stellar properties of YSES 2 in Table sition, Li absorption, X-ray emission, and the confirmed LCC
1. In §4.2 and §4.3, we derive updated stellar parameters for membership clearly favor the pre-main-sequence evolutionary
YSES 2, more importantly, including stellar mass and age. stage.
4.1. Previous studies 4.3. Updated stellar parameters
YSES 2 was first identified as a young star in the Search for As- Using the VOSA spectral energy distribution (SED) analyzer
sociations Containing Young stars (SACY) survey (Torres et al. (Bayo et al. 2008),2 we fit synthetic stellar spectra to the ob-
2006) of optical counterparts to the ROSAT All-Sky Survey served visible and infrared photometry for YSES 2. For priors,
(RASS) X-ray sources (Voges et al. 1999). Torres et al. (2006) we constrained the reddening to be E(B − V) = 0.016 ± 0.017
mag based on the STILISM 3D reddening maps (Lallement et al.
reported the star to be a Li-rich (EW[Li I λ6707] = 367 mÅ)
2019) and searched for best-fit synthetic spectra in the range
K1V(e) star with filled-in Hα, showing fast rotation (v sin i =
3000 K < T eff < 6000 K, 3.5 dex < log g < 4.5 dex, and metallici-
19.3 km s−1 ) and radial velocity 15.8 ± 1.0 km s−1 . Based on its
ties -0.5 < [Fe/H] < 0.5 and [α/Fe] = 0.0. A Bayesian fit using the
position, proper motion, and youth indicators, Preibisch & Ma-
BT-Settl-CIFIST models using 22 photometric points yielded the
majek (2008) included the star in a list of new members of the
following parameters: AV = 0.06 (0.02-0.09; 68%CL; 0.00-0.11;
LCC subgroup of the Sco-Cen OB association (their Table 4),
95%CL), T eff = 4749 K (4709-4789 K; 68%CL; 4700-4900 K;
and provided initial estimates of isochronal age (16 Myr), mass
95%CL), log g = 3.9 dex (3.5-4.5 dex), [Fe/H] = 0.0. We present
(1.1 M ), and fractional X-ray luminosity (log(LX /Lbol ) = -3.2).
the results of this SED fit in Fig. 1.
Preibisch & Mamajek (2008) also predicted a kinematic distance
The best-fit bolometric flux is fbol = (1.7490 ± 0.0248) ×10−9
of 109 pc (based on the proper motion and space velocity of
erg s−1 cm−2 , which on the IAU 2015 apparent bolometric mag-
LCC), which compares remarkably well to the Gaia EDR3 paral-
lactic distance ($ = 9.1537 ± 0.0118 mas, D = 109.25 ± 0.14 pc; 2
Online available at: http://svo2.cab.inta-csic.es/theory/
Gaia Collaboration et al. 2020). Kiraga (2012) reported the star vosa/index.php
Article number, page 3 of 15A&A proofs: manuscript no. ajbohn_yses_companion_tyc8984
Table 1. Stellar properties of YSES 2. 1.0 10.0
10 8 Photometry 10 8
Parameter Value Ref. Tycho
10 9 APASS 10 9
Main identifier YSES 2 (1) SDSS
TYCHO ID TYC 8984-2245-1 (2) Gaia
10 10 DENIS 10 10
F [erg/s/cm2/ m]
2MASS ID J11275535-6626046 (3) 2MASS
WISE
GAIA EDR3 ID 5236792880333011968 (4) 10 11 10 11
α (J2000) [hh mm ss.sss] 11 27 55.355 (4)
δ (J2000) [dd mm ss.ss] -66 26 04.50 (4) 10 12 10 12
Spectral Type K1V(e) (5)
10 13 10 13
$ [mas] 9.1537 ± 0.0118 (4)
D [pc] 109.25 ± 0.14 (7) 10 14 10 14
BT-Settl-CIFIST: Teff = 4750 K, log(g) = 3.9 dex, [Fe/H] = 0.0
µα∗ [mas yr−1 ] -34.025 ± 0.013 (4)
1.0 10.0
µδ [mas yr−1 ] 2.319 ± 0.011 (4) [ m]
vrad [km s−1 ] 13.41 ± 0.17 (5)
Fig. 1. Spectral energy distribution of YSES 2. The colored markers
B [mag] 11.819 ± 0.010 (8) indicate the archival photometric measurements of the star and the blue
V [mag] 10.860 ± 0.017 (8) curve presents our best-fit BT-Settl-CIFIST model to the data. The un-
G [mag] 10.525 ± 0.003 (4) certainties of the photometric measurements are too small to be visual-
ized in the figure.
I [mag] 9.773 ± 0.044 (9)
J [mag] 9.006 ± 0.026 (3)
H [mag] 8.484 ± 0.029 (3)
Ks [mag] 8.358 ± 0.029 (3)
W1 [mag] 8.323 ± 0.014 (10) majek 2016)) and benefits from a very precise distance, well-
W2 [mag] 8.351 ± 0.008 (10) constrained extinction from 3D reddening maps, and integrat-
W3 [mag] 8.258 ± 0.019 (11) ing synthetic SEDs using 22 photometric data points. Combining
this improved luminosity estimate with the improved T eff from
W4 [mag] 7.929 ± 0.118 (11) the SED fitting (T eff = 4749 ± 40 K) yields a good estimate of
Prot [day] 2.7325 (9) the radius of the star (1.193 ± 0.022 R ). A comparison against
v sin i [km s−1 ] 19.3 ± 0.5 (5) evolutionary tracks from Baraffe et al. (2015) provided an up-
log(LX /Lbol ) [dex] -3.07 ± 0.23 (9) dated stellar mass of (1.10 ± 0.03) M and an age of (13.9 ±
EW(Li I λ6707) [mÅ] 364 ± 0.05 (6) 2.3) Myr. Furthermore we note that the SED of the star showed
no signs of infrared excess through to the WISE-4 band (22 µm).
EW(Hα) [mÅ] 0.0 (5) The 2MASS-WISE colors (K s -W1 = -0.035 ± 0.032 mag, K s -
U [km s−1 ] -10.10 ± 0.08 (7) W2 = -0.007 ± 0.030 mag, K s -W3 = 0.100 ± 0.035 mag, K s -W4
V [km s−1 ] -18.93 ± 0.12 (7) = 0.429 ± 0.122 mag) can be compared to the mean for K1 pre-
W [km s−1 ] -5.60 ± 0.09 (7) main-sequence stars from Pecaut & Mamajek (2013) (K s -W1
AV [mag] 0.06+0.03 (6) = 0.09 mag, K s -W2 = 0.06 mag, K s -W3 = 0.10 mag, K s -W4 =
−0.04
0.18 mag), and show a significant hint of IR excess. The K s -
T eff [K] 4749 ± 40 (6) W4 color is marginally red (2σ excess), perhaps hinting at a
mbol [mag] 10.396 ± 0.015 (6) debris disk (common among non-accreting pre-main-sequence
Mbol [mag] 5.204 ± 0.016 (6) Sco-Cen stars), but there is no corroborating evidence to further
log (L/L ) [dex] -0.1854 ± 0.0063 (6) support this.
R [R ] 1.193 ± 0.022 (6)
Mass [M ] 1.10 ± 0.03 (6) We searched for common proper motion stellar or substel-
Age [Myr] 13.9 ± 2.3 (6) lar companions for YSES 2. Assuming a mass of 1.0 M , the
tidal radius of YSES 2 is ∼1.35 pc (projected radius ∼0◦ .71 or
References. (1) This paper, see §2; (2) Høg et al. (2000); (3) Cutri et al. ∼256000 ; Mamajek et al. 2013), that is, bound companions would
(2003); (4) Gaia Collaboration et al. (2020), and Gaia EDR3 and DR2 be expected to lie projected within this radius. Surveying the lists
ID #s are the same; (5) Torres et al. (2006); (6) this paper, see §4.3; of Sco-Cen candidates and pre-main-sequence stars from the
(7) distance and heliocentric Galactic Cartesian velocity calculated us- Gaia DR2 catalogs of Goldman et al. (2018), Zari et al. (2018),
ing Gaia EDR3 values (D = 1/$); (8) Henden et al. (2016); (9) Kiraga and Damiani et al. (2019) with parallactic distances ofAlexander J. Bohn et al.: Discovery of a directly imaged planet to the young solar analog YSES 2
5. Observational results and analysis measurements at higher spectral resolution. Whereas the masses
of the spectrally similar trio of HR 8799 c, d, and e are in the
In §5.1 we show that our observations reveal a co-moving com- range 7-12 MJup (Wang et al. 2018; Marois et al. 2008, 2010), it is
panion to YSES 2. The reduced images for both epochs are pre- likely that YSES 2b has an even lower mass as the system age of
sented in Fig. 2. Our photometric analysis in §5.2 indicates that (13.9±2.3) Myr is significantly younger than the age of HR 8799,
this companion has a mass that is significantly lower than the which is claimed to be member of the Columba association with
deuterium burning limit of ∼ 13 MJup . We refer to this newly an age of 30–50 Myr (Zuckerman et al. 2011; Bell et al. 2015).
identified planet as YSES 2b henceforth. In §5.3 we present the This is supported by the AMES-COND and AMES-dusty mod-
detection and mass limits of our acquired data. els (Allard et al. 2001; Chabrier et al. 2000) that we present in
Fig. 4 for a system age of 13.9 Myr. An individual evaluation of
5.1. Companion astrometry these isochrones yielded masses from 5.3 MJup to 8.0 MJup as
presented in Table 2. The uncertainties originate from the errors
We extracted the companion astrometry and photometry by the in the system age and planet magnitude that were propagated by
injection of negative artificial companions (e.g., Lagrange et al. a bootstrapping approach with 1,000 randomly drawn samples
2010; Bonnefoy et al. 2011). A detailed description of our from Gaussian distributions around both parameters. When com-
method is presented in Appendix C. The extracted astrometry bining the posterior distributions for the different models and fil-
is listed in Table 2. As visualized in the proper motion plot ters we derived a final mass estimate of 6.3+1.6
−0.9 MJup as the 68%
in Fig. 3 the companion is clearly incompatible with the cal- confidence interval around the median of the sample. This esti-
culated trajectory of a static background object at 14σ signifi- mate is based on broadband photometric measurements alone;
cance. The relative astrometric motion with respect to the pri- further spectral coverage of the planetary SED will be important
mary is consistent with a comoving companion. This conclusion to constrain its effective temperature, luminosity, surface gravity,
is further confirmed by a similar analysis of other point sources and mass.
within the detector field of view. As presented in Appendix D,
all additional off-axis point sources are consistent with being
non-moving background contaminants. We thus conclude that 5.3. Detection limits
YSES 2b is a gravitationally bound companion to its solar-mass
host star. From our astrometric measurements we derived a pro- To derive upper mass limits for additional companions in the sys-
jected physical separation of approximately 115 au. Future as- tem, we calculated the detection limits of our datasets. As a base-
trometric measurements are required to derive meaningful con- line, we evaluated the contrast in the image that was obtained
straints to the orbital parameters of this wide-orbit planet. by de-rotating and median combining the individual exposures
without any PSF subtraction. This image covers the full field of
view of the SPHERE/IRDIS detector up to an angular separation
5.2. Companion photometry of 500. 5. We evaluated the contrast directly in the final image us-
ing aperture photometry. The chosen aperture size was one full
We present the photometry of the companion in Fig. 4 in a width at half maximum (FWHM) of the unsaturated stellar PSF
color-magnitude diagram. The corresponding numerical values as measured in the flux images (see Table A.1 in Appendix A).
are reported in Table 2. YSES 2b is consistent with a late L The signal flux was measured as the sum over the full circular
to early T spectral type when comparing it to colors of field aperture within the mean combined flux image and scaled for
brown dwarfs from the NIRSPEC Brown Dwarf Spectroscopic the flux difference with the science frames owing to the shorter
Survey (McLean et al. 2003, 2007), the IRTF spectral library exposure times and the applied neutral density filters. For sev-
(Rayner et al. 2009; Cushing et al. 2005), the L and T dwarf data eral radial positions that were equidistantly sampled from 000. 15
archive Knapp et al. (2004); Golimowski et al. (2004); Chiu et al. to 500. 50 in steps of 000. 05, we measured the noise as the standard
(2006), and the SpeX Prism Libraries (Burgasser et al. 2010; deviation of the integrated flux within apertures that were dis-
Gelino & Burgasser 2010; Burgasser 2007; Siegler et al. 2007; tributed around the star at the same radial separation (excluding
Reid et al. 2006; Kirkpatrick et al. 2006; Cruz et al. 2004; Bur- the signal aperture itself). We applied a sigma clipping with an
gasser & McElwain 2006; McElwain & Burgasser 2006; Shep- upper bound of 3σ to the integrated fluxes of the noise apertures
pard & Cushing 2009; Looper et al. 2007; Burgasser et al. 2008; before calculating the standard deviation to discard apertures that
Looper et al. 2010; Muench et al. 2007; Dhital et al. 2011; Kirk- were polluted by flux of off-axis point sources (see the full frame
patrick et al. 2010; Burgasser et al. 2004). Object distances were image in Fig. D.1 in Appendix D). A correction for small sample
derived from Gaia EDR3 (Gaia Collaboration et al. 2020), the statistics as described in Mawet et al. (2014) was considered in
Brown Dwarf Kinematics Project (Faherty et al. 2009), and the these noise calculations. We reiterated this analysis for six uni-
Pan-STARRS1 3π Survey (Best et al. 2018). In color-magnitude formly spaced position angles and present the azimuthally aver-
space, YSES 2b is very close to the innermost three planets of aged results as two-dimensional contrast curves in Fig. 5. The
the HR 8799 multi-planetary system (Marois et al. 2008, 2010). solid yellow lines represent the 5σ raw contrast in the H and K s
These three planets are classified as mid to late L type dwarfs bands that was obtained without any PSF subtraction.
(e.g., Greenbaum et al. 2018), which agrees well with the se-
For the innermost region around the star (< 100. 2), the sensi-
quence evolution of the adjacent field brown dwarfs from L to
tivity was additionally assessed considering our PSF subtraction
T spectral types.3 A similar spectral type in this domain, there-
by RDI plus PCA. We used the ContrastCurveModule from
fore, seems very likely for YSES 2b, requiring confirmation by
PynPoint version 0.6.04 that utilizes the same aperture pho-
3
We would like to note that the planets around HR 8799, although
4
closely located to the sequence of field brown dwarfs in the selected As mentioned before, version 0.8.1 of PynPoint was used for all re-
SPHERE filters as presented in the color-magnitude diagram in Fig. 4, maining analysis steps. The modules of both versions are compatible;
can have near-infrared colors in different passbands that are signifi- only the implementation of some algorithms changed throughout the
cantly distinct from those of their field dwarf analogs (e.g., Currie et al. development process. This affects the ContrastCurveModule, which
2011). follows the iterative process described in this paragraph for release ver-
Article number, page 5 of 15A&A proofs: manuscript no. ajbohn_yses_companion_tyc8984
Fig. 2. Multi-epoch observations of YSES 2 and its planetary-mass companion. Final data products of the SPHERE observations collected in the H
band (left panel) and K s band (right panel) are presented. For both filters, the stellar PSF is modeled by 50 principal components that were derived
from a reference library of YSES targets. These PSF models were subtracted and the residuals rotated such that north points up and east toward the
left. In the presented images the median of these de-rotated residuals is shown. For the K s band data, uncorrected residuals of a wind-driven halo
are detected that extend from the northeast to southwest. The planet YSES 2b is highlighted by white arrows. The primary is located at the origin
of the coordinate system and we artificially masked the inner region up to the radial extent of the coronagraphic mask of 100 mas. To assess the
spatial extent of the instrumental PSF, the median combination of the non-coronagraphic flux images of the primary star are shown in the lower
left of each panel. The intensity of each flux image is rescaled to match the maximum and minimum counts in the corresponding residual science
image, and we display both images with the same spatial and color scales.
Table 2. Astrometry, photometry, and derived masses of YSES 2b.
Observation date Filter Separation Position angle ∆Mag Mabs Photometric mass
COND Dusty
(yyyy-mm-dd) (mas) (◦ ) (mag) (mag) (MJup ) (MJup )
2018-04-30 H 1057 ± 3 205.3 ± 0.2 10.37 ± 0.11 13.57 ± 0.11 5.3 ± 0.5 8.0 ± 0.7
2020-12-08 Ks 1053 ± 5 205.2 ± 0.2 9.55 ± 0.11 12.71 ± 0.11 6.1 ± 0.6 6.4 ± 0.6
tometry framework and metric to evaluate the contrast for sev- for each of the injection positions. For the calculation of the final
eral positions that were distributed around the star in the resid- contrast curves with RDI plus PCA, we used a radial sampling
ual images. For each position, the module injects an artificial in the range 000. 15-100. 20 with a spatial resolution of 000. 05, and six
companion, whose detection significance is evaluated after the position angles that were equidistantly sampled in polar space.
PSF subtraction with RDI combind with PCA. In this frame- Again, the contrast as a function of radial separation was ob-
work, the signal aperture is directly placed on top of the posi- tained by azimuthal averaging of the various position angles. We
tion at which the artificial companion has been injected, and the considered several numbers of principal components to model
noise apertures are azimuthally distributed around the primary the stellar PSF as indicated by the sequentially colored, dashed
star as described before, yet excluding the signal aperture itself. lines in Fig. 5. The detectable planet masses that correspond to
The companion template was obtained as the median combina- the calculated magnitude contrasts were derived by evaluation of
tion of the non-coronagraphic flux images that was scaled for AMES-COND models at the system age of 13.9 Myr (see right
the difference in exposure time and the attenuation due to the axes of Fig. 5).
applied neutral density filter. From an initial magnitude contrast The contrast performance close the star improves for an in-
of 8 mag, the flux of the injected companion was adjusted and creasing number of principal components. This differential gain
the post-processing was performed iteratively until the artificial in contrast ceases for ∼50 subtracted components and the con-
companion was retrieved at 5σ detection significance in the final trast for 100 principal components does not change significantly
image product. These limiting magnitude contrasts were stored compared to the curve generated for half as many components.
sion 0.6.0. We prefer this implementation over the solution presented This justifies our previously selected value of 50 principal com-
in PynPoint version 0.8.1, which calculates one attenuation factor per ponents that were used for our PSF subtraction with RDI plus
position that is based on the single injection of an artificial companion PCA. This amount of components is equivalent to 19 % and 30 %
with a user-defined signal-to-noise ratio. of the reference libraries in the H and K s bands, which are com-
Article number, page 6 of 15Alexander J. Bohn et al.: Discovery of a directly imaged planet to the young solar analog YSES 2
2020-12-08 if background
Fig. 3. Proper motion plot for YSES 2b. The colored markers represent
the relative astrometry with respect to the primary star measured for our
two observational epochs. The blue trajectory indicates the simulated
motion of a static background object at infinity and the white marker
is the theoretical position of such an object at our second observational
epoch.
HIP 64892 B
HIP 79098 (AB)b
YSES 1b
1 RXS 1609 b
AB Pic B
β Pic b
HIP 65426 b 10
7 Fig. 5. 5σ detection limits of our SPHERE/IRDIS observations in the
9
PDS 70 b H (upper panel) and K s bands (lower panel). On the left axis the magni-
5 8 tude contrast with respect to the primary star is reported, and the abso-
HR 8799 e lute magnitudes are converted to detectable planet masses with AMES-
HR 8799 d YSES 1c
COND models as indicated on the right axis; this scale varies between
HR 8799 c
HR 8799 b the H and K s bands. The solid yellow lines represent the limits when
3
no PSF subtraction is performed. The dashed lines indicate the sensitiv-
ity, when a PSF subtraction with RDI plus PCA is performed. The red
51 Eri b star highlights the contrast of YSES 2b that we detect at ∼ 5σ signifi-
cance in both filters after PSF subtraction with more than 50 principal
components.
Fig. 4. Color-magnitude diagram for YSES 2b. The colored markers nitudes deeper than our raw data. At angular separations larger
show the sequence of field brown dwarfs with various spectral types than 100 the contrast improvement decreases as the flux con-
from M to late T. The white markers represent known directly imaged
companions that are usually younger than the presented field objects.
tribution of the stellar PSF becomes negligible. At separations
YSES 2b is highlighted by the red star. We further show AMES-COND ≥ 200 we reach a fundamental noise floor that is mainly com-
and AMES-dusty evolutionary models that were evaluated at a system posed of residual sky background and detector read out noise.
age of 13.9 Myr (solid lines). The markers along the line indicate the The K s band contrast behaves very similar to the detection lim-
equivalent object masses in MJup . its in the H band and the RDI plus PCA reduction scheme can
provide a maximum gain of up to 2.5 mag at an angular separa-
tion of ∼000. 4. The overall improvement for separations < 100 is
posed of 269 and 164 individual frames, respectively. In the H marginally worse compared to the H band data and the contri-
band we observe a contrast improvement of more than two mag- bution of the asymmetric wind-driven halo is clearly visible for
nitudes at an angular separation of 000. 2. This corresponds to an separations in the range 000. 5-100. 2. Combining the data from the
increase in planet detection sensitivity by more than 45 MJup at H and K s bands allows us to exclude stellar and brown dwarf
this close separation. The contrast improvement in the H band companions around YSES 2 with masses > 13 MJup for angular
is maximized at an angular separation of ∼000. 3, where RDI plus separations that are larger than 000. 15. At 000. 5 we are sensitive to
PCA provides detection limits that are approximately three mag- objects that are more massive than 6 MJup and for angular sepa-
Article number, page 7 of 15A&A proofs: manuscript no. ajbohn_yses_companion_tyc8984
rations that are larger than 200 we can even rule out planets with nearly ubiquitous (see, e.g., Garufi et al. 2018 for an overview,
masses as low as 2 MJup . observed in scattered light). These structures can, in a growing
This demonstrates, impressively, how a large reference li- number of cases, be traced out to tens or hundreds of Astro-
brary can help to significantly improve the contrast performance nomical Units (e.g., ALMA Partnership et al. 2015; Ginski et al.
at small angular separations < 100. 2. Especially for datasets with 2016; de Boer et al. 2016; van Terwisga et al. 2018). Recently it
little parallactic rotation, RDI plus PCA should be considered as was found that these substructures are already present in proto-
a default PSF subtraction strategy. This conclusion is also sup- stellar disks as young as ∼0.1 Myr (Sheehan et al. 2020; Sheehan
ported by first results from the star-hopping mode that was re- 2020), suggesting that planet formation sets in early and operates
cently implemented at VLT/SPHERE (Wahhaj et al. 2021). As on short timescales. Despite this abundance of substructures ob-
visualized by the red stars in Fig. 5, RDI plus PCA is required served in young, circumstellar environments, it is unlikely that
to detect YSES 2b at 5σ significance in both the H and K s band all these protoplanetary disks are gravitationally unstable and
data. support planet formation via this channel (e.g., Kratter & Lodato
2016). Conversely, we expect that the timescale to form a planet
via core accretion at the current location of YSES 2b would be
6. Discussion
too long, given the system age and that the gas-rich disk in the
The newly discovered planetary companion to YSES 2 is among system has already dissipated (e.g., Haisch et al. 2001).
the lowest mass direct imaging companions known to date. The Even though capturing scenarios are considered unlikely for
only objects of similar low mass are 51 Eri b (Macintosh et al. YSES 2b, we cannot confidently conclude whether its forma-
2015), HD 95086 b (Rameau et al. 2013), HR8799 b (Marois tion via either top-down or bottom-up scenarios is more likely:
et al. 2008), PDS 70 b and c, (Keppler et al. 2018; Haffert et al. whereas a mass of 6.3 MJup is rather low for an object to orig-
2019; Wang et al. 2020; Stolker et al. 2020a), and YSES 1c inate from gravitational disk instabilities, core accretion would
(Bohn et al. 2020b). Of these, only YSES 1c is located around favor a formation at closer separations to the star. More data are
a solar-mass star. Within the uncertainties, the mass of YSES 2b thus required to explore the origin of this wide-orbit Jovian giant.
is the same as YSES 1c, which we previously discovered in our Promising methods to evaluate the likelihood of either forma-
survey. Given our mass estimate for the planet, the mass ratio of tion scenario are by characterization measurements of the plan-
YSES 2b to its host star is q = 0.54+0.13−0.08 %. This value is com- etary atmosphere, continuous orbital monitoring to constrain es-
parable, but slightly lower than the q = 0.57 ± 0.10 % derived pecially its eccentricity, and deeper searches for additional com-
for YSES 1c. The mass ratio is the lowest among direct imaging panions in the system.
companions to solar-type stars.5 To identify the formation channel of YSES 2b via atmo-
The in situ formation of super-Jovian planets at tens or hun- spheric characterization, we can utilize the framework postulated
dreds of Astronomical Units is challenging. We recently dis- by Öberg et al. (2011), who argued that elemental abundances in
cussed possible formation scenarios for such objects in the con- the planetary atmosphere (and especially the C/O ratio) are di-
text of the YSES 1 system in Bohn et al. (2020a). In our pre- rectly linked to its natal environment in the planet-forming disk.
vious study we considered scattering or planet capturing events Different ice lines in the protoplanetary disk and the associated
to explain the current large separation of YSES 1b. However, freeze out of the corresponding molecular species alter molecu-
dynamic scattering by a third body in the system is expected lar abundance ratios as a function of radial separation from the
to produce high eccentricities, inconsistent with orbital stabil- host star, making this atmospheric quantity a promising indica-
ity of both planetary components in the YSES 1 system (Bohn tor of the natal environment and formation channel of a planet.
et al. 2020b).6 The new detection of YSES 2b in our small survey The chemical and dynamical evolution of the disk can alter these
sample of 70 solar-type systems in Sco-Cen makes the hypoth- initial abundances and should be considered in the analysis (e.g.,
esis that we see captured free-floating planets unlikely. Goulin- Ali-Dib et al. 2014; Mordasini et al. 2016; Eistrup et al. 2016,
ski & Ribak (2018) find with numerical simulations that only 2018). Gravity Collaboration et al. (2020) utilized this frame-
∼0.1% of solar-type stars in the Galactic thin disk should cap- work to study β Pic b and proposed its formation through core
ture a free-floating planet in their lifetime. YSES 2b in principle accretion, with strong planetesimal enrichment based on its sub-
might have formed in situ via disk gravitational instability. Boss solar C/O abundance ratio. If YSES 2b formed via disk grav-
(2011) find that they can produce 1-5 MJup planets between 30 au itational instability, then we expect this object to have similar
and 70 au with eccentricities as high as 0.35. If YSES 2b is on elemental abundances as the primary star in the system, while
an eccentric orbit it may explain its current projected separation formation by core accretion should lead to an over-abundance
of 110 au. Kratter et al. (2010) conversely find with their hy- of heavy elements due to pebble accretion. YSES 2b, along with
drodynamic simulations that planets formed via disk instability other planet mass objects detected by direct imaging, provides an
need to be at large separations outside of the 40 au to 70 au range ideal test case for future detailed atmospheric characterization.
to not accumulate too much mass and remain in the planetary We can further continue to monitor the separation and po-
regime. This hypothesis that gravitationally instabilities predom- sition angle of YSES 2b with respect to the primary star to de-
inantly create brown dwarf and stellar companions is supported rive orbital solutions for the planet (e.g., Wang et al. 2018). In
by other theoretical studies (e.g., Zhu et al. 2012; Forgan & Rice particular, the VLTI/GRAVITY instrument (Gravity Collabora-
2013). Spatially resolved observations of gas-rich planet form- tion et al. 2017) will be extremely useful for this purpose, as it
ing disks in the last few years have shown that radial substruc- facilitates an unprecedented astrometric precision down to sub-
tures, which are thought to be caused by perturbing planets, are milliarcsecond scales (e.g., Gravity Collaboration et al. 2019;
5
See Bohn et al. (2020a) for an overview of mass ratios of direct imag-
Wang et al. 2021). These astrometry measurements could be
ing companions to solar-type stars. complemented by VLT/CRIRES+ data to constrain the radial ve-
6
We, however, note that in some cases high eccentricities are inferred locity of the planet and to obtain three-dimensional information
for substellar companions, for example, the brown dwarf companion about its orbital motion (e.g., Schwarz et al. 2016). The eccen-
to the young solar analog PZ Tel (Mugrauer et al. 2012; Ginski et al. tricity of the planet might provide hints regarding the likelihood
2014). of a potential migration of the companion, which would be an
Article number, page 8 of 15Alexander J. Bohn et al.: Discovery of a directly imaged planet to the young solar analog YSES 2
indicator of formation via core accretion at closer separation to The mass and separation of YSES 2b are inconsistent with
the star. If this migration was caused by scattering off another, planet populations for most in situ formation scenarios: whereas
so far undetected companion to the primary star, a deep imaging disk instabilities predominantly create companions above the
campaign is required to search for evidence of such an additional deuterium burning limit at a separation of 110 au, core-accretion
component to the planetary system. At the moment, we cannot mechanisms are not efficient enough to form a planet of 6.3 MJup
provide conclusive evidence for the most likely formation sce- this widely separated from the primary star. So, the new com-
nario of YSES 2b based on the available data; but future obser- panion might be either at the low-mass end of potential in situ
vations might be able to shed light on the origin of this Jovian formation outcomes from top-down scenarios, or it formed via
gas giant. core accretion at closer separation to the star and migrated to its
Even though the second epoch observations of YSES are current location. Atmospheric characterization measurements of
not concluded yet and candidate companions to ∼45 stars of molecular abundance ratios, orbital monitoring, and evaluation
our sample need to be confirmed or rejected, our survey has al- of the eccentricity of the planet, or a deep search for additional
ready discovered three planetary-mass companions amongst 70 companions in the system, might help to evaluate the likelihood
young, Sun-like stars. This high planet-detection rate is in stark of these potential formation pathways. While we cannot rule out
contrast to previous surveys that were targeting Sun-like stars at scattering or capture scenarios, we point out that the former re-
closer distances than the LCC (e.g., Kasper et al. 2007; Biller quire an (as of yet) third undetected body in the system, while
et al. 2013; Galicher et al. 2016), which discovered mostly stel- the latter are unlikely given numerical simulations. YSES 2b is
lar and brown dwarf companions. These preliminary statistical an important addition to the sparsely populated group of wide-
results from YSES tentatively indicate that despite the farther orbit gas giant companions. Owing to the moderate separation
distance, Sco-Cen and especially LCC are more favorable than with respect to the primary star, spectroscopic observations with
moving groups in the immediate solar neighborhood for the de- JWST, VLT/ERIS, or VLTI/GRAVITY will be easily available.
tection of young planets briefly after their formation. Many of These data will be important to further constrain the properties of
the moving groups that were targeted in these aforementioned this Jovian companion. Measurements of molecular abundance
surveys are significantly older than LCC (15 ± 3 Myr), such as ratios such as C/O or its orbital eccentricity might even facilitate
the Tucana-Horologium moving group (45 ± 4 Myr Bell et al. hypotheses regarding the most likely formation mechanism for
2015), the AB Dor moving group (149+51 −19 Myr; Bell et al. 2015),
this wide-orbit gas giant planet.
or the Hercules-Lyra association (257 ± 46 Myr; Eisenbeiss et al. Our data rule out brown dwarf and stellar companions with
2013). As a consequence of the decreasing luminosity of objects M > 13 MJup in the SPHERE/IRDIS field of view for angular
below the deuterium burning limit with increasing age (e.g., Bur- separations > 000. 15 and at 000. 5 we can exclude objects that are
rows et al. 1997), it is natural that the sensitivity to Jovian plan- more massive than 6 MJup . At separations that are larger than
ets is worse around members of these associations compared to 200 we are even sensitive to planets with masses as low as 2 MJup
significantly younger host stars. Yet some of these closer mov- . In general, the applied PSF subtraction scheme based on RDI
ing groups have ages comparable to that of LCC – such as the plus PCA is extremely successful and provides substantial con-
TW Hya association (10 ± 3 Myr) or the β Pic moving group trast improvements (> 1 mag) for separations that are smaller
(24 ± 3 Myr; Bell et al. 2015) – and should provide even bet- than 100 . In the H band, the PSF subtraction enhances our sensi-
ter planet-detection sensitivities owing to their much closer dis- tivity by more than 45 MJup at 000. 2, and the greatest contrast im-
tances. However, before speculating about potential reasons for provement of ∼3 mag is achieved at an angular separation of 000. 3.
this tentative overabundance of planetary-mass companions to Our YSES strategy with short snapshot observations of ≤ 5 min
our YSES targets, it is necessary to finish the second epoch ob- combined with a large reference library for PSF subtraction is
servations of the survey and to derive reliable occurrence rates certainly a promising approach to image planetary-mass com-
of planetary-mass companions to Sun-like stars in Sco-Cen. panions to young, Sun-like stars in Sco-Cen. With three newly
discovered planetary-mass companions in less than 40 h of al-
located telescope time the survey efficiency is unprecedented
7. Conclusions and the mission concept can certainly be applied to future high-
contrast imaging studies targeting different samples of pre-main-
We report the detection of a new directly imaged planet to the sequence stars.
solar-mass primary YSES 2 that was discovered within the scope
of YSES. Reassessment of the stellar parameters provided an Acknowledgements. We would like to thank the anonymous referee for the very
effective temperature of T eff = (4749 ± 40) K, a luminosity of valuable feedback that helped improving the quality of the manuscript. Espe-
cially, the extremely kind way of providing this feedback was highly appreciated
log (L/L ) = −0.1854 ± 0.0063, a mass of (1.10 ± 0.03) M , and by the authors. The research of AJB and FS leading to these results has re-
a system age of (13.9 ± 2.3) Myr. ceived funding from the European Research Council under ERC Starting Grant
We detect YSES 2b in two consecutive epochs collected on agreement 678194 (FALCONER). Part of this research was carried out at the
2018 April 30 and 2020 December 18 with VLT/SPHERE. The Jet Propulsion Laboratory, California Institute of Technology, under a contract
with the National Aeronautics and Space Administration (80NM0018D0004).
companion has a projected separation of approximately 100. 05, MM would like to thank the German Research Foundation (DFG) for support
which translates to a physical minimum distance of ∼115 au with in the program MU 2695/27-1. CA acknowledges support by ANID – Millen-
respect to the primary star. Photometric measurements in the H nium Science Initiative Program – NCN19_171. MR acknowledges support from
and K s bands constrain a planet mass of 6.3+1.6
−0.9 MJup according
the FWO research program under project 1280121N. This research has used the
SIMBAD database, operated at CDS, Strasbourg, France (Wenger et al. 2000).
to AMES-COND and AMES-dusty evolutionary models. This This work has used data from the European Space Agency (ESA) mission Gaia
mass estimate is supported by the position of the object in color- (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Process-
magnitude space, where it is located amongst the mid to late L ing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/
type field brown dwarfs and close to HR 8799 c, d, and e. The gaia/dpac/consortium). Funding for the DPAC has been provided by na-
tional institutions, in particular the institutions participating in the Gaia Mul-
slightly higher mass estimates of these exoplanets on the order of tilateral Agreement. This publication makes use of VOSA, developed under the
7–12 MJup are consistent with the older system age of HR 8799 Spanish Virtual Observatory project supported by the Spanish MINECO through
of 30–50 Myr. grant AyA2017-84089. To achieve the scientific results presented in this article
Article number, page 9 of 15A&A proofs: manuscript no. ajbohn_yses_companion_tyc8984
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(Foreman-Mackey et al. 2013), scikit-image (Van der Walt et al. 2014), scikit- Forgan, D. & Rice, K. 2013, MNRAS, 432, 3168
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Gaia Collaboration, Brown, A. G. A., Vallenari, A., et al. 2018, A&A, 616, A1
Gaia Collaboration, Brown, A. G. A., Vallenari, A., et al. 2020, arXiv e-prints,
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